A
photovoltaic solar electric panel generates DC power
when it is exposed
to sunlight. A natural question to ask is 'How much power?' Here we
explain
how to test a solar panel for output and how to determine the maximum
power
point.

A
solar
panel is a generator of DC electrical power. DC
electrical power
wattage is the product of voltage and current.

DC
Watts
= Voltage X Current

Power
is
measured in units of Watts, (Symbol W)

Voltage
is measured in units of Volts, (Symbol V)

Current
is measured in units of Amps, (Symbol I)

The
power
generated by any solar panel in full sunlight
depends on the resistance
of the electrical load connected to it. Resistance is measured in units
of
Ohms. The relationship between current, voltage and resistance is
referred
to as Ohm's law. The current through a resistor is the applied voltage
divided
by the resistance.

Current
=
Voltage / Resistance

The
method
of measuring the power output of a solar
panel is to connect resistors
of various values to the panel and measure the voltage. The
measurements
can be used to calculate the power output. The same measurements can be
used
to plot the power output and create a performance graph for the panel.

A
digital
multimeter for measuring DC voltage is
required for this test.
An assortment of power resistors is also needed. In practice, each
power
resistor is connected to the panel and the voltage is measured with the
meter,
as shown in the drawing.

The
meter
is also used to measure the open circuit
voltage and the short
circuit current. Here is a table of data from a set of measurement on 3
large
solar panels connected in series. In this example power resistors of
100,
50, 25 and 3 ohms were used. The current through each
resistor is
calculated by dividing the measured voltage by the resistance.
The
power is calculated by multiplying the voltage times the current.

Note
that
no power is generated when the open
circuit voltage and short
circuit current is measured. A more intuitive view of the
data
can
be gained by plotting a graph of the voltage versus current.

The
output
of this solar panel shows a characteristic
behavior that is common
to all solar panels. The maximum power is generated at the
operating
point which forms the 'knee' in the curve. In our case, this is at
approximately
81 watts, corresponding to a load resistance of 25 ohms. The maximum
power
point is where the product of current and voltage is a maximum.
Expressed
graphically, the maximum power point it where the largest area
rectangle
can be formed beneath the curve.

Testing
a
solar panel requires an assortment of power
resistors capable of
handling the power. In most cases, ordinary 1/4 and 1/2 watt
resistors
are NOT suitable for this type of measurement. Small resistors overheat
and
burn out. Power resistors are made to handle more wattage, although
they
will become hot over time and can burn out too. Adjustable power
resistors
(called rheostats) are also useful, although they require a
meter
for
measuring the resistance after each adjustment. The simplest way to
avoid
problems with overheating is to use high wattage resistors and to make
individual
measurements as quickly as possible.

The
power
output of solar panels can be increased by
tracking the position of the sun, as described in our Solar Tracker E-Booklet.

Electric
DC
gearhead motors can be used as a DC
generator, and can be tested in a similar way, as described in our Motor Generator E-Booklet.

Special
Offer:
Power Resistor
Assortment

MTM
Scientific offers an assortment of POWER
RESISTORS (CAT# POWER)
for testing small solar electric panels. Our assortment contains 10
different
fixed power resistors, each with a 5 Watt rating. The assortment also
includes
an adjustable 200 ohm power rheostat rated for 50 Watts. The
fixed
resistance values included are: 820, 100, 50, 27, 10, 5, 2, 1, .5 and
.1
ohms. Each power resistor is individually labeled with the
resistance
value to make testing quick, easy and accurate. The cost of this item
is
$23.50 and it is sent postage paid in the USA. Please click the link
below
to purchase using Paypal. International
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